This invention relates generally to electrical control circuits and, more particularly, to an improved frequency sensitive switching circuit for controlling the energization of loads such as ballasted fluorescent and high intensity discharge lamps.
The above-referenced Foehn patent describes a load control system particularly useful for selectively controlling banks of ballasted lamps in a manner facilitating the implementation of energy conservation measures. More specifically, the system permits the ballasted loads to be selectively disconnected from a power circuit without disturbing other loads connected to the circuit and without substantial modification of existing wiring. Control signals having respective preselected frequencies are applied to the power circuit conductors at a convenient location remotely of the loads. Frequency sensitive switching circuits connect the loads to the conductors, and these switching circuits are actuated in response to the control signals to energize only the desired loads.
Briefly, each of the frequency sensitive switching circuits used in this system comprises a solid state switching device, such as a triac, having first and second main terminals and a control gate for controlling the conductance between the terminals. The first main terminal of the triac is connected to one of the AC power circuit conductors which supply power to the load, while the second main terminal is connected to one side of the load, the other side of the load being connected to the neutral conductor of the AC power circuit. An impedance element, such as a resistor or a parallel resonant circuit, is connected between the control gate and the first main terminal of the triac, and a series resonant circuit adapted to pass the control signal and block the operating power is connected between the control gate and the neutral AC power conductor.
In the absence of a control signal having a frequency at which the series resonant LC circuit is tuned, the gate circuit will not be activated and the triac remains nonconducting. Hence, if the load comprises one or more ballasted fluorescent lamps, the section of light system controlled by this triac switching circuit will remain turned off. In order to energize this section of the lighting system, a remotely located frequency generator is activated to superimpose on the power line conductors a control signal having a frequency matching that to which the above-mentioned LC resonant circuit is tuned. Since the series resonant circuit will pass the control signal, the full control signal appears across the gate connected impedance element, causing the triac to turn on and energize the load. In order to keep the triac conducting and maintain energization of the load, the gate circuit of this prior art frequency sensitive switch must be continuously activated by the control signal. Once the control signal is terminated, the triac will be turned off, and the load will be de-energized. Hence, although the load control system of the aforementioned Foehn patent represents a significant advance in the art with respect to energy conservation, the advantages of the system could be significantly enhanced if it was not necessary to continuously consume signal power in order to maintain load energization.
The aforementioned application Ser. No. 912,606, Hidler and Plumb, provides an improved frequency sensing switching circuit which significantly reduces the consumption of control signal power in a comparatively simple and economical manner. More specifically, the switching circuit of the Foehn patent is modified as follows. The junction of the capacitor and inductor of the series resonant circuit is connected directly to the triac terminal which is coupled to the load. Further, an additional series capacitor is connected between the resonant circuit inductor and the neutral power circuit conductor. The capacitance value of this additional series capacitor is selected to block the operating power and pass the control signal having a frequency matching that at which the series resonant circuit is tuned. As a result of this circuit modification, the transmitted control signal is developed across the gate impedance means to actuate the triac into conduction at the end of each half cycle of operating power. The resulting conduction of operating power through the switching device is then operative to effectively short out the capacitor component of the series resonant circuit and thereby cause the inductor component of the resonant circuit to block the control signal for the remainder of the operating power half cycle. Hence, the control signal is blocked during all but a small portion of each half cycle of the applied AC power, thereby significantly reducing the consumption of control signal power.
Although the above-described circuit of the Hidler and Plumb application satisfactorily provides the desired power conserving operation, the constant switching function involved can deteriorate the triac and, thus, affect the operating life of the circuit.